1. Field of the Invention
The present invention relates to a resist composition for use in the step of producing a semiconductor such as IC, in the production of a circuit board for liquid crystal, thermal head and the like, and in the lithography step of other photofabrication processes, and also relates to a pattern formation method using the composition. More specifically, the present invention relates to a resist composition suitable for exposure by an immersion-type projection exposure apparatus using a light source of emitting far ultraviolet light at a wavelength of 300 nm or less, and a pattern formation method using the composition.
2. Description of the Related Art
Along with the refinement of semiconductor devices, the trend is proceeding toward a short wavelength exposure light source and a high numerical aperture (high NA) projection lens. At present, exposure machines with NA of 0.84 have been developed, where an ArF excimer laser having a wavelength of 193 nm is used as the light source. As commonly well known, these can be expressed by the following formula:(Resolving power)=k1·(λ/NA)(Depth of focus)=±k2·λ/AN2 wherein λ is the wavelength of exposure light source, NA is the numerical aperture of projection lens, and k1 and k2 are constants related to the process.
In order to obtain shorter wavelength and higher resolving power, studies are being made on exposure machines where an F2 excimer laser having a wavelength of 157 nm is used as the light source. However, the lens material used for the exposure apparatus and the material used for the resist so as to obtain shorter wavelength are very limited and therefore, it is very difficult to stabilize the production cost or quality of the apparatus and materials. This may lead to a failure in procuring the exposure apparatus and resist having sufficiently high performance and stability, within a required time period.
With regard to the technique of elevating the resolving power in optical microscopes, a so-called immersion method of filling a high refractive index liquid (hereinafter sometimes referred to as an “immersion liquid”) between the projection lens and the sample has been conventionally known.
As for the “effect of immersion”, assuming that λ0 denotes the wavelength of exposure light in air, n denotes the refractive index of immersion liquid to air and θ denotes the convergence half-angle and that NA0=sin θ, the above-described resolving power and depth of focus when immersed can be expressed by the following formulae:(Resolving power)=k1·(λ0/n)/NA0 (Depth of focus)=±k2·(λ0/n)/NA02 
That is, the effect of immersion is equal to use of an exposure wavelength of 1/n. In other words, in the case of a projection optical system of the same NA, the depth of focus can be made n times larger by the immersion. This is effective for all pattern profiles and furthermore, can be combined with super-resolution techniques being studied at present, such as phase-shift method and modified illumination method.
Examples of the apparatus applying this effect to the transfer of a fine pattern of semiconductor devices include those described in JP-A-57-153433 and JP-A-7-220990, but these are not discussing on the resist suitable for immersion exposure techniques.
JP-A-10-303114 indicates that the control of refractive index of the immersion liquid is important because changes in the refractive index of immersion liquid bring about deterioration of the projected image due to difference in the wave front aberration of exposing machine, and discloses to control the temperature coefficient of refractive index of the immersion liquid to a certain range or use, as a suitable immersion liquid, water where the surface tension is decreased or an additive of increasing the surface activity is added. However, the additive is not disclosed or the resist suitable for immersion exposure techniques is not discussed.
Recent progress of immersion exposure techniques is reported, for example, in Proceedings of Society of Photo-Optical Instrumentation Engineers (Proc. SPIE), Vol. 4688, page 11 (2002) and J. Vac. Sci. Tecnol. B, 17 (1999). In the case of using an ArF excimer laser as the light source, in view of safety on handling and transmittance and refractive index at 193 nm, pure water (refractive index at 193 nm: 1.44) is considered most promising as the immersion liquid. In the case of using an F2 excimer laser as the light source, a fluorine-containing solution is being studied in the light of balance between transmittance and refractive index at 157 nm, but those satisfied from the aspect of environmental safety or in the refractive index have been not yet found out. The immersion exposure technique is considered to be most soon mounted on an ArF exposing machine in view of the degree of immersion effect and the maturity of resist.
Since the discovery of the resist for KrF excimer laser (248 nm), an image formation method called chemical amplification is used as the image formation method for the resist so as to compensate the reduction of sensitivity due to light absorption. To describe this by taking as an example the image formation method using positive chemical amplification, an acid generator in the exposed area decomposes upon exposure to generate an acid, the acid generated is used as a reaction catalyst in the post-exposure baking (PEB) to convert the alkali-insoluble group into an alkali-soluble group, and the exposed area is removed by alkali development.
In the immersion exposure, the resist film is exposed through a photomask in the state of an immersion liquid being filled between the resist film and the optical lens, and thereby the pattern of the photomask is transferred to the resist film. It is estimated that the immersion liquid permeates into the inside of the resist film and affects the chemical reaction (e.g., acid catalyst-type deprotection reaction, development reaction) brought about inside the resist during or after exposure. However, its degree of effect or mechanism is not yet elucidated.
When a chemical amplification resist is applied to the immersion exposure technique, the acid on the resist surface generated upon exposure moves to the immersion liquid and the acid concentration on the surface of exposed area is changed. This may be considered to have a close resemblance to the acid deactivation occurring on the surface of exposed area due to basic contamination in a very small amount on the order of several ppb mingled from the environment at the post-exposure time delay (PED), which is a serious problem at the initiation of development of the chemical amplification-type positive resist, but the effect of immersion exposure on the resist or the mechanism is not clearly known. When a chemical amplification-type resist causing no problem in the lithography by normal exposure is exposed by the immersion method, there arises a problem that the depth of focus (hereinafter sometimes referred to as “DOF”) as a tolerance for focus fluctuation and the profile are deteriorated.